Strip chart recorders are often used to record temperature or other environmental conditions during the transport of cargoes. Prior recorders have included a strip of pressure-markable chart paper having an inner end wound into a supply roll and an outer end forming a leader that extends across a marking station to a takeup reel. A battery-powered motor slowly turns the takeup reel while a stylus at the marking station presses against the strip to mark it. When the shipment is delivered, the person receiving it breaks a seal to open a housing of the chart recorder and removes the chart from the takeup reel. The person can examine the chart to determine whether, for example, the temperature during transport has exceeded a limit which might indicate spoiled goods. The chart is valuable in establishing whether or not the carrier is responsible for certain damages to the goods.
Such prior chart recorders had many disadvantages. One disadvantage is that the chart was subject to a "wrap factor" wherein the first portion of the strip to be recorded upon moved slowly because the windup roll on the takeup reel had a small diameter, while the last portion of the chart moved at a higher rate because the windup roll then had a large diameter. The chart could be printed with variable spaces between time markings to try to account for this variation in chart speed, but the compensation will not be accurate. The "wrap factor" was an important source of error in determining, for example, precisely when the goods were subjected to high temperature or shock. In prior mechanisms the strip chart sometimes was not properly aligned, which resulted in wrinkling of the chart during its rollup. Also, the chart was difficult to review because it tended to curl up due to having been wound onto a small diameter takeup reel.
After the chart is retrieved, the chart recorder may be returned to the manufacturer for installation of another chart for reuse; alternatively, if there is damage to the shipment the mechanism may be retained for possible litigation. Prior chart recorders have often been packaged in reclosable plastic containers having hinged covers, to facilitate access to the chart with minimum possibility of damage to the mechanism during retrieval of the chart, and to facilitate shipment to the manufacturer. If would be desirable if the chart could be retrieved without having to open the housing, as this would permit a lower cost housing to be used and minimize the possibility of damage to the mechanism.